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CDC: West Nile virus cases continuing to accumulate

Thursday September 13, 2012
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This year’s West Nile virus outbreak is on pace to be the most lethal on record, according to the Centers for Disease Control and Prevention.

As of Sept. 11, the CDC had received reports of 2,636 cases of West Nile virus disease in people, with 118 deaths. Those numbers increased about 35% from the previous week’s report.

"We’re still largely monitoring cases that occurred several weeks ago, and that was the peak of the epidemic," said Lyle R. Petersen, MD, MPH, director of the Division of Vector-borne Infectious Diseases in the National Center for Emerging and Zoonotic Infectious Diseases. "So we still expect large numbers of cases to be coming in, even for the next several weeks, even though the epidemic itself or the number of infections may be going down at this point."

Of the cases, 1,405 (53%) were classified as neuroinvasive disease such as meningitis and encephalitis. As was the case the previous three weeks, the number of neuroinvasive cases is the highest number reported through this time of year since the West Nile virus was detected in 1999. The total of number cases is the highest reported since 2003.

"We consider neuroinvasive disease the best indicator of the scope of the epidemic, since these cases are most consistently reported, and thus we continue to believe that this year’s outbreak is the most serious [West Nile] outbreak to date," Petersen said.

As was the case in previous weeks, two-thirds of the reported cases are in Texas, Louisiana, South Dakota, Mississippi, Michigan and Oklahoma, with 40% in Texas.

"We know that West Nile virus outbreaks are highly focal," Petersen said. "It depends on a number of ecological factors which are hard to measure, and which are influenced by weather … such as the number of birds, susceptible birds in an area or conditions that may have caused more mosquitoes or change the interaction of mosquitoes and birds.

"All of this is something that we’re going to try and sort out in the upcoming weeks and months, as we start to look at the data in more detail."

The CDC wants to investigate other issues as well, Petersen said. "The infection’s only been here for 13 years, and so we really want to know what the long-term public health impacts and impacts on people are." The CDC also plans to examine "the genetic characteristics of the virus that circulated this year and see if there’s any change in this virus that may have altered the course of the epidemic. We do not believe that is the case, but we just want to make sure … that the virus, let’s say, is not more virulent than it used to be."

The CDC also addressed reports of a shortage of West Nile virus test kits available to state health departments, possibly leading to delayed diagnoses. The CDC confirmed a short-term shortage, but said it has been resolved and laboratories should be able to meet their needs for the remainder of the season.

The CDC’s tips for reducing the risk of contracting West Nile virus include using repellents when outdoors, emptying standing water from items outside the home (such as gutters, flowerpots, buckets, kiddie pools and birdbaths), wearing long sleeves and pants during dawn and dusk, installing or repairing screens on windows and doors, using air conditioning if possible, and supporting local community mosquito control programs.


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